Ca2+-mediated remote control of reversible sieve tube occlusion in Vicia faba

According to an established concept, injury of the phloem triggers local sieve plate occlusion including callose-mediated constriction and, possibly, protein plugging of the sieve pores. Sieve plate occlusion can also be achieved by distant stimuli, depends on the passage of electropotential waves (EPWs), and is reversible in intact plants. The time-course of the wound response was studied in sieve elements of main veins of intact Vicia faba plants using confocal and multiphoton microscopy. Only 15-45 s after burning a leaf tip, forisomes (giant protein bodies specific for legume sieve tubes) suddenly dispersed, as observed at 3-4 cm from the stimulus site. The dispersion was reversible; the forisomes had fully re-contracted 7-15 min after burning. Meanwhile, callose appeared at the sieve pores in response to the heat shock. Callose production reached a maximum after approximately 20 min and was also reversible; callose degraded over the subsequent 1-2 h. The heat induction of both modes of occlusion coincided with the passage of an EPW visualized by electrophysiology or the potential-sensitive dye RH-414. In contrast to burning, cutting of the leaf tip induced neither an EPW nor callose deposition. The data are consistent with a remote-controlled occlusion of sieve plates depending on the longitudinal propagation of an EPW releasing Ca(2+) into the sieve element lumen. It is hypothesized that forisome plugs and callose constriction are removed once the cytosolic calcium level has returned to the initial level in those sieve tubes.